This invention generally relates to calibration apparatus and methods and more particularly relates to a calibration arrangement and method for calibrating an inspection instrument, such as an ultrasonic inspection instrument of the kind typically used to inspect nuclear reactor pressure vessels.
Although calibration devices and methods are known in the prior art, it has been observed that prior art devices and methods have certain operational problems associated with them which make these devices and methods unsuitable for calibrating inspection instruments of the kind typically used to inspect nuclear reactor pressure vessels. However, before these problems can be appreciated, some background is necessary as to the structure, operation and inspection of a typical nuclear reactor pressure vessel.
In this regard, a nuclear reactor pressure vessel is a device for producing heat by controlled fission of fissionable material contained in a reactor core disposed in the pressure vessel. Pressurized liquid moderator coolant (i.e., borated demineralized water) is caused to circulate through the pressure vessel, by means of inlet and outlet nozzles welded thereto, and through the reactor core for assisting in the fission process and for removing the heat produced by fission of the fissionable material contained in the reactor core. The heat provided by the reactor core is ultimately carried by the coolant to a turbine-generator set for providing revenue-producing electricity in a manner well known in the art of nuclear powered electricity generation.
However, there is a remote possibility that during its service life, the material comprising the pressure vessel may indicate anomalies. Such anomalies, if they were to occur, may be due, for example, to neutron radiation embrittlement and/or the corrosive effects of the borated moderator coolant. Therefore, ASME (American Society of Mechanical Engineers) Code, Section XI recommends that reactor pressure vessels be inspected for anomalies during the service life of the pressure vessels.
It is current practice to perform this inspection using ultrasonics. During the inspection, an ultrasonic inspection device is moved by remote means over a portion of the interior of the pressure vessel, such as over the pressure vessel interior wall, nozzles and weldments, to detect the size and severity of any anomalies therein. However, the operating response or output signals of the ultrasonic inspection device may vary on a daily basis or even hourly due to "drift", instrument component aging, or the like. Variation in the operating response of the ultrasonic inspection device may lead to inaccurate inspection results unless the inspection device is periodically re-calibrated by appropriate means.
To perform the initial calibration, a so-called "calibration block" is used to calibrate the ultrasonic inspection device. The calibration block of current practice comprises a five to nine inch thick and relatively heavy steel block having at least one hole therein to simulate an anomaly of the type that may be encountered in the pressure vessel (e.g., the pressure vessel wall, nozzles and weldments). In this regard, an ultrasonic transducer is brought into contact with and moved on the surface of the calibration block to direct ultrasonic energy in the direction of the hole, which is located at a predetermined distance or depth within the block. The steel material of the block serves as the sound transmission or sonic coupling medium that sonically couples the hole to the ultrasonic transducer, so that a response is obtained from the hole. More specifically, the ultrasonic transmission, which may be either a shear wave or longitudinal wave sound transmission, is introduced into the block at refracted angles of between 0 degrees to 70 degrees. A return echo with a characteristic amplitude is produced when the sonic energy encounters the hole. The return echo is detected by the ultrasonic transducer, which generates a signal indicative of the depth and orientation of the hole in the steel block. In this manner, the calibration block calibrates the ultrasonic device so that it is capable of suitably alerting the test operator when similarly shaped anomalies are detected in the pressure vessel. Once the initial calibration of the system has been established, the initial calibration is usually verified or validated by re-calibration at approximately 12-hour intervals during the pressure vessel examination because, as previously mentioned, the response of the device may vary over time. In the prior art, this validation or re-calibration effort is customarily performed on the previously mentioned steel calibration block in the same manner as the original calibration.
However, it is current practice to place the calibration block externally to the pressure vessel. This necessitates that, during the process of pressure vessel examination, the ultrasonic transducer be remotely retrieved from the pressure vessel and placed on the calibration block. Each retrieval of the ultrasonic transducer requires that nonessential maintenance personnel in the vicinity of the pressure vessel leave the area as the transducer is removed from the pressure vessel in order to avoid radiation exposure to the maintenance personnel. If they were to remain in the vicinity of the pressure vessel as the transducer is retrieved from the vessel, such personnel would be exposed to radiation because the transducer is radioactively contaminated during its tenure in the pressure vessel. These personnel re-enter the vicinity of the pressure vessel after the transducer is reintroduced into the vessel following re-calibration because once the transducer is inside the pressure vessel, the pressure vessel and liquid moderator therein shield the personnel against radiation emanating from the radioactively contaminated transducer. However, such time consuming exit and re-entry of maintenance personnel results in nonproductive or lost time which in turn increases maintenance costs because such maintenance personnel are not performing maintenance activities while away from the pressure vessel. Therefore, a problem in the art is to perform the required re-calibration or validation in a manner not necessitating the exit and re-entry of such maintenance personnel, so that maintenance costs are reduced.
Also, the time dedicated to retrieving the transducer from and reintroducing the transducer into the pressure vessel may delay returning the reactor to service, if the inspection is performed on the critical path for completion of all pressure vessel maintenance activities. Any such delay in returning the reactor to service may result in lost revenue of approximately $1,000,000 per day for the utility owner. Therefore, a problem in the art is to perform the validation in a more time-efficient and hence cost-effective manner that obviates the need to retrieve and then reintroduce the transducer into the vessel, so that there is no delay in returning the pressure vessel to service.
Moreover, certain essential maintenance personnel required to perform the original calibration and subsequent validation must remain in the vicinity of the pressure vessel. Such essential maintenance personnel may be exposed to low doses of radiation during the re-calibration or validation process. Although such radiation doses are within acceptable limits, it is nonetheless desirable to lower the level of such radiation doses. Therefore, another problem in the art is to perform the re-calibration or validation in a manner that lowers radiation doses to such essential maintenance personnel.
Therefore, what is needed is a calibration arrangement and method for calibrating an inspection instrument, such as an ultrasonic inspection instrument of the kind typically used to inspect nuclear reactor pressure vessels.